1. Field of the Invention
The present invention relates to an apparatus for ablating an object by laser beam and an ablating method using the ablation apparatus, and more particularly to an apparatus for ablating the surface of a cornea by laser beam to correct the refractive power and an ablating method thereof.
2. Description of Related Art
There are conventionally known processing apparatuses for ablating an object by laser beam. A laser beam used mainly for fine processing, is the excimer laser, particularly argon fluoride (ArF) laser with a wavelength 193 nm. The use of high harmonic wave, such as Nd:YAG laser, and others have also been tried. A known fine processing apparatus is an apparatus for ablating a surface of the cornea of an eye to change curvature thereof and thereby correct a refractive error of the eye. In such ablating apparatus, it is important to control the ablating operation so that the ablated area is uniform in depth.
Therefore, various ablating methods to ablate an object uniformly in depth are proposed. In Japanese Patent Application No. 2-416767 corresponding to U.S. patent application Ser. No. 07/812,819, the inventor of the present invention also has proposed an ablating apparatus, that is, an apparatus for ablating a surface of a cornea by excimer laser beam having a uniform intensity distribution in one direction and a Gaussian distribution in a vertical direction thereof, and scanning optical elements in the Gaussian direction, so that the ablated area is uniform in depth.
In one constructed apparatus for ablating a surface of a cornea with uniform depth, the laser beam irradiating area on a cornea is restricted by a variable aperture and a variable width parallel slit, and the irradiating area is controlled to form a cornea shape with a desired curvature, to correct refractive error of the cornea including myopia, astigmatism and others.
However, ablating depth per pulse (per scan in a method described below in the embodiment) subtly varies according to primary factors such as kinds of objects to be ablated, output energy of the laser beam and other processing conditions. The ablating depth per one scan of the laser beam is hereinafter called "ablation rate" in the present specification. It may be considered that the ablation rate by excimer laser is substantially the same when the laser is continuously worked, but the ablation rate often changes when processing conditions are different, even if the output energy of the laser beam is the same. Such change of ablation Pate is fatal to a processing apparatus which needs close control of ablating depth, particular in an apparatus for forming a cornea into a desired regular shape. Due to a changed ablation rate, the cornea is formed into a shape different from the desired one.
It is generally difficult to measure the ablation rate, in particular the ablation rate of a transparent object such as a cornea.
Even if the measurement of the ablation rate itself can be conducted, by an electron microscope and the like, measuring the ablation rate every ablation is troublesome in operation thereof and too expensive.